This invention relates to metal alloys and particularly to Fe-Al alloys which can be used in high temperature oxidation environments, such as in a catalytic converter for controlling automotive emission gases. The development of high temperature metal alloys, resistant to oxidation at such temperatures, first used nickel or cobalt based super alloys as well as austenitic stainless steels. In an effort to lower cost and thereby increase applications of such metals, the prior art proceeded to use less rich iron based alloys containing either aluminum, chromium or silicon. These binary alloys were less costly and did provide some degree of oxidation resistance but failed to provide proper oxidation resistance at temperatures in the range of 500.degree.-800.degree. C. and under long time or cyclic high temperature excursions.
For example, the binary alloy of iron and aluminum offers protection which is due to the preferential oxidation of the aluminum to form an aluminum oxide film. This film tends to protect the alloy surface from further oxidation, but when there is a break in the film, such binary alloy is subject to failure because of the growth of iron oxide nodules in generally the 500.degree.-800.degree. C. range. It is generally believed that the growth of the aluminum oxide film depleted the subjacent iron alloy of aluminum so that when a break did occur in the protective oxide film, the iron rich substrate was directly exposed to the atmosphere and iron oxide nodules began to form. Above 800.degree. C., failure occurs by a mechanism of pealing of the protective oxide. Increasing the aluminum concentration within a binary iron alloy tends to suppress the iron oxide nodule formation, but above eight percent, the alloy becomes disadvantageously unworkable in the cold mill.
Each of the above-mentioned binary alloys possess some disadvantage that prompts further development. As indicated, the iron/aluminum alloy produce catastrophic oxidation failure at temperatures above 500.degree. C. The iron/chromium alloys with the high chromium content, such as are well known in stainless steels, are simply too expensive. The iron/silicon alloys suffer from processing problems associated with the formation of a low melting iron/silicon/oxygen phase (fayalite) when reheated for hot working. The fayalite drips off the ingots and slabs and fills up the furnace bottoms much faster than the solid scale that is formed thereon.
The prior art, in its search to provide a low cost, high temperature resistant metal, proceeded to use one or more additional active elements to back up aluminum in a binary alloy. The additional elements, such as chromium or silicon, act as a getter of oxygen, preventing diffusion of the oxygen into the alloy once a break occurred in the primary protective oxide film. This prevented growth of the iron oxide nodule, but even these ternary alloys proved to be disadvantageous because they had a limited suitability under sustained high temperature exposure and particularly under cyclic heating and cooling conditions wherein thermal shock resulted in flaking and spalling of the oxide coating. What is needed is an alloy composition which lends itself to low cost manufacture and at the same time offers high temperature resistance performance which is not affected by cyclic heating conditions which cycle from a temperature below 600.degree. C. to above 900.degree. C., and preferably 1000.degree. C.